During operation, optical elements or optical assembly groups in optical arrangements can become contaminated when contaminating substances are deposited on their optical surfaces. This problem occurs in an Extreme Ultra-Violet (EUV) lithography systems, in which for the generation of EUV radiation a laser beam of a drive laser system is directed at a target position of a target material. The target material is converted to a plasma state when irradiated with the laser beam and thereby emits EUV radiation. Such an EUV lithography system is described, for example, in U.S. Pat. No. 8,173,985 B2. Upon irradiation with the laser beam, a portion of the target material (e.g., tin) is typically vaporized and is deposited on the optical surfaces of optical elements arranged in the vicinity of the target position.
To counteract this problem, a CO2 laser is generally used as the drive laser of such an optical arrangement. Owing to its long wavelength of about 10.6 micrometers, CO2 laser radiation is also reflected by reflective optical elements which have a comparatively rough optical surface, as is caused by tin deposits. In the case of some target materials, for example tin, the use of a drive laser in the form of a CO2 laser additionally permits a high conversion efficiency between the input power of the drive laser and the output power of the EUV radiation.
In the above-described EUV lithography system, and also in other optical arrangements, it may from time to time be necessary to carry out maintenance work on the optical elements or the optical assembly groups, for example to clean the optical elements or replace them with uncontaminated optical elements. If an optical assembly group is arranged in a closed housing (e.g., in a vacuum housing), it can generally only be dismantled with a relatively high outlay. If direct access to the optical assembly group, for example by maintenance openings, is not possible due to the available space, it is necessary to remove the optical module or the optical assembly group from the housing to carry out the maintenance work.
Removing the optical module from the housing gives rise to the problem that, when the maintenance work is complete the optical module (more precisely the optical elements of the optical module) must be re-positioned with high accuracy relative to the optical elements that have remained in the housing. In the case of a vacuum housing, there may also be deformation of the housing walls caused by the vacuum, which makes correct positioning of the optical elements, or of the optical module, more difficult. A complex and tedious adjustment is therefore typically necessary for the correct positioning of the optical elements of the optical module when the maintenance work is complete.
The EUV radiation generated in an optical arrangement is fed to an illumination system (not shown) to illuminate as homogeneously as possible an image field on which a structured element (mask) is arranged. A projection system (also not shown) serves to reproduce the structure of the mask on a light-sensitive substrate (wafer). The beam-generation system, the illumination system and the projection system together form an EUV lithography apparatus for the structuring of the light-sensitive substrate (wafer).
The system and methods described herein can advantageously provide an optical arrangement having an optical module and a housing which facilitate the correct positioning of the optical elements of the optical module in the housing.